The industry body said it welcomed the inclusion of in-house professionals in the register along with consultants, among proposals announced on Friday.
But it said the proposed register would only include "significant" lobbying of MSPs, while lobbying civil servants and special advisers would not require registration.
Any lobbying not deemed "significant" in terms of time spent or cost would not be recorded by the Registrar.
The proposals also suggest that those who appear on the register would adhere to a non-binding code which mirrors the MSP Code of Conduct.
Francis Ingham, PRCA director-general, said: "Given that 80 per cent of our industry is in-house, we welcome the fact the committee has avoided the UK Government’s dire mistake and actually included them in their proposal.
"Holyrood is leading the way here and Westminster could learn a great deal. While this is a sound opening gambit, there are flaws which must be corrected at the earliest possible opportunity. The fact it only covers MSPs completely ignores the great deal of work done with civil servants. That would be a fundamental misinterpretation of our industry."
Ingham said PRCA members would need greater clarity over what did or did not constitute "significant" lobbying activity and that the current proposals were "arbitrary and difficult to define".